1. Field of the Invention
The invention relates to the field of switched mode power converters and in particular to a high power, DC-to-DC converter utilizing a phased array of lower power converters operating in parallel.
2. Description of the Prior Art
Switched mode power converters accomplish various forms of power conversion through systematic switching and storage of electrical energy. A wide variety of circuit topologies are used to address many different power conversion requirements and applications. The regulation of the power conversion process is generally implemented through the control of one or more active switching devices.
Most circuit topologies used in DC-to-DC switched power mode converter technology use a control technique which monitors and samples one or more critical operating parameters, processes this parameterized information, and produces a single output which is used to drive a single active power switching device or several power switching devices which are connected in series or parallel. There are also several different topologies which use two control outputs to drive two or four active power switching devices.
Switched mode power conversion is commonly used because it is more efficient than linear regulation, which dissipates unused power as a primary method for regulation. Theoretically, if the switches and energy storage elements were lossless, that is lossless inductors and capacitors, switched mode power conversion would be 100 percent efficient. In reality, because components are not ideal and do exhibit loss, the efficiency is less than 100 percent, typically in the range of 75 to 95 percent depending on the circuit topology and the application.
Switched mode power conversion is also commonly used because as the switching frequency increases, the size and weight of the converter is reduced. This reduction in size and weight is due primarily to the frequency scaling properties of the energy's storage components.
One of the inherent shortcomings of switched mode power conversion becomes apparent in applications which require smaller sizes and higher frequencies. The smaller size requirements dictate that the switching frequency of the converter must be increased dramatically. Often, the increase in frequency goes beyond the most efficient operating range of the components available. Consequently, parasitic effects begin to dominate and losses increase, which cause a reduction in the conversion efficiency and a significant increase in the complexity of the circuit design. This relationship between size and frequency ultimately limits the size and efficiency of switched mode power converters.
Size and frequency limitations become even more significant as power level increases. This is because the larger components required to convert the high power levels generally exhibit a higher degree of parasitics, losses and nonideal behavior. In addition, these effects usually occur at lower frequencies when the size of the components increase. Therefore, it becomes more difficult to increase the switching frequency when the amount of power which must be converted increases.
Another problem which arises from the reduction in size and increase in frequency is the creation of high operating temperatures and hot spots. This is because of the lumped nature of the circuit topologies and also because the reduction in size is not accompanied with the corresponding reduction in losses or increases in efficiency. In fact, as mentioned above, the operating efficiency tends to decrease in this situation. Therefore, heat removal becomes a significant problem.
The use of parallel phase shifted DC-to-DC converters is known in the art. See Hergenhan, U.S. Pat. No. 4,290,101 "N Phase Digital Converter," (1981). However, in Hergenhan, the control circuit separately provide a sequential triggering pulse to each of the power switches in a manner to provide nonoverlapping control pulses as depicted in Hergenhan's FIG. 2.
The switching of flyback transformers to lower RMS values of ripple current through the input and output capacitors of a power conversion circuit is shown in Petersen, U.S. Pat. No. 4,972,292 "Inductor with Centertap Switching Transistor for Reduced Radio Frequency Emissions," (1990). Petersen, however, describes a four-switch array in which each of the switches are driven 90 degrees out of phase with respect to each other in a nonoverlapping relationship with varying duty cycles as shown in lines A and B of Peterson's FIG. 2a.
A parallel array of frequency converters for an AC power source is shown by Harada et al., "X-Ray Power Supply with Plural Frequency Converters," U.S. Pat. No. 5,105,351 (1992). However, although the primary coils of the switched transformers are in parallel, their secondary output circuits are ganged in series to provided a boosted up DC output voltage.
Inou et al., "DC/DC Converter," U.S. Pat. No. 4,685,039 (1987), describes a DC-to-DC converter having switched primaries coupled in series and secondary circuits coupled in parallel to the output capacitance and load. The embodiment of FIG. 7b of Inou shows three such circuits coupled in parallel between the input and output capacitances. Switching of the primary circuit in each circuit is either simultaneous as shown by Inou's FIG. 4a or 180 degrees out of phase as shown in FIG. 4b. No sequential phase shifting among a plurality of switched primaries is considered beyond the two modes.
Therefore, what is needed is some type of switched mode power converted topology and methodology by which each of the shortcomings relating to size and frequency limitations, particularly in a high power application, is avoided or reduced.